1. Field of the Invention
The present invention relates to an electromagnetic wave detector made of semiconductor material with a quantum well structure. The detector operates optimally in the range of frequencies between 1 and 5 Terahertz (THz) (corresponding to wavelengths between 300 and 60 microns) although operation at higher or lower frequencies is included.
2. Description of the Prior Art
Semiconductor epitaxy enables one to grow adjacent layers of semiconductor with different band gaps. FIGS. 1 a-d show a few examples of the kinds of quantum wells that can be grown using semiconductor epitaxy. The solid lines represent the energy of the conduction band as a function of position, which is proportional to the concentration x of Al in Al.sub.x Ga.sub.1-x As. The dashed lines represent the energy levels, or subbands, that are allowed for an electron occupying the various quantum wells. The lowest subband is called the ground state. Higher lying subbands are called excited states. FIG. 1(A) shows a relatively narrow square well in which the two lowest subbands are separated by a relatively large energy. FIG. 1(B) shows a wider square well in which the two lowest subbands are separated by a much smaller energy. FIG. 1(C) shows two square wells which are coupled to each other by tunneling through a thin barrier. In this well, the first two subbands are separated by an energy small compared to the spacing between second and third subbands. FIG. 1(D) shows a quantum well in which the band gap is continuously graded over the width of the quantum well.
The promotion of an electron from one subband to another is called an intersubband transition. A photon can induce an intersubband transition if its frequency matches the spacing between the ground state and first excited state. It is possible to place electrons in quantum wells by adding impurities to, or doping the semiconductor. FIG. 2A shows a doped quantum well, in which the impurities have been added in the region of low band gap (for example, GaAs). This has the advantage of being simple but the disadvantage that the electrons scatter off the impurities as they move through the quantum well. FIG. 2B shows a remotely-doped quantum well, in which the impurities are placed in a region with high band gap fairly close to the quantum well. The electrons populate the well because that is the region where they have the least potential energy. In a remotely-doped quantum well, the electron-impurity scattering is much weaker than in a doped quantum well.
Semiconductor epitaxy offers the opportunity to design improved detectors for THz radiation. The potential of epitaxy has been realized in the mid-ranges of infrared (2-18 .mu.m wavelengths) by quantum well infrared photoconductors (QWIPs). See B. F. Levine et.al., "New 10 mu m infrared detector using intersubband absorption in resonant tunneling GaAlAs superlattices!," Applied Physics Letters 50 (16), 1092-4 (1987). In a QWIP a photon which is resonant with an intersubband transition promotes an electron e to an excited state, and that electron is collected as shown in the energy diagram of FIG. 3. Bois et. al. "Electromagnetic Wave Detector with Quantum Well Structure.", U. S., Pat. No. 5,506,418, (1996) describes a two-terminal bolometer based on intersubband transitions (ISBTs). The ability to tune the intersubband absorption frequency by applying a voltage between gate and drain has been demonstrated by Heyman et. al. "Resonant harmonic generation and dynamic screening in a double quantum well," Physical Review Letters 72 (14), 2183-6 (1994) and is shown in FIG. 4. A tuning range of up to a factor of four has been achieved. The ability to place a "back gate" on a sample, which is a conducting layer between quantum well and substrate, has been proven by Hopkins et. Al. "Logarithmically graded quantum well far-infrared modulator," Applied Physics Letters 64 (3), 348-50 (1994). Hopkins et. al. have also demonstrated the disposition of a layer of so-called low-temperature grown (LT) GaAs between the back gate and the quantum well in order to electrically isolate the quantum well from the back-gate. LT GaAs is insulating by virtue of a high concentration of defects. It is also possible to isolate the back gate from the quantum well with a layer of semi-insulating GaAs, which is grown at a temperature near 200.degree. C., followed by annealing near 600.degree. C. After the annealing process, an LT-GaAs layer contains inclusions of As. It is thought 20 that these inclusions result in its highly insulating character. It is also possible to isolate the back gate from the quantum well with an epitaxial layer of semi-insulating Al.sub.x Ga.sub.1-x As, which is insulating by virtue of being not intentionally doped, thus having a low carrier concentration of impurities and free carriers.
Several antenna-coupled detectors for THz radiation based on semi-conductors have been proposed. Norton, "Integrated IR and MM-Wave Detector", U.S. Pat. No. 5,432,374 (1995) has proposed an antenna-coupled FET. In this detector, "mm-wavelength radiation that is received by the antenna structure causes a modulation of current flow between source and drain." Fedichkin et.al. "A novel tunable infrared detector based on a quantum ballistic channel," Infrared Physics 34 (5), 477-80 (1993) have proposed an antenna coupled to a quantum point contact, yielding a voltage tunable detector for THz radiation. See also, B.Xing et.al., "Novel voltage-tunable far-infrared and terahertz detector based on a quantum ballistic channel," Semicond. Sci. Technol. 10, 1139 (1995). Keay et al. "Dynamic localization, absolute negative conductance, and stimulated, multiphoton emission in sequential resonant tunneling semiconductor superlattices," Physical Review Letters 75 (22), 4102-5 (1995) have demonstrated an antenna-coupled diode in which radiation received by the antenna is driven through the diode, and the resulting rectified voltage or current is detected.
The detectors of radiation which prior to the present invention exhibited the best combination of speed and sensitivity over a broad band of frequencies near the THz range are superconducting hot-electron bolometers. See A. Skalare et.al. "Large bandwidth and low noise in a diffusion-cooled hot-electron bolometer mixer," Applied Physics Letters 68 (11), 1558-60 (1996). Skalare demonstrates that it is well known within the art to use a hot-electron bolometer as a mixer. Operated as mixers, these detectors have reached noise temperatures of 650 K, which is approximately 25 times the fundamental limit h.upsilon./k.sub.B imposed by quantum mechanics at 533 GHz with an IF bandwidth of 1.7 GHz corresponding to a response time of 100 psec.
In the Terahertz frequency range, frequency resolution is usually obtained using some sort of mechanical spectrometer or interferometer, such as a Michelson or a Fabry-Perot interferometer, in combination with a detector which is sensitive over a broad band of frequencies.
Notwithstanding the foregoing prior art there is much room for improvement in detectors and spectrometers for Terahertz radiation. Outstanding problems include:
(1) the Bulk, weight, and complexity of spectrometers: Typical spectrometers add unwanted bulk, weight and complexity to space-based and other nonlaboratory applications.
(2) Coupling efficiency: A critical problem with all antenna- or waveguide-coupled detectors is to maximize the efficiency of coupling to the absorbing region. This is accomplished by matching the impedance of the absorbing region to that of the antenna. In superconducting hot-electron bolometers, the impedance of the absorbing region is fixed upon fabrication. Some electrical network is required to match the impedance of the antenna to the impedance of the bolometer. See for example the prior art impedance transforming element for a superconducting bolometer shown by J. Mees et.al., "New designs for antenna-coupled superconducting bolometers," Applied Physics Letters 59 (18), 2329-31 (1991). For a particular matching network, impedance-matching is achieved only over a narrow band of frequencies.
(3) Overloading by background radiation: Detectors which are sensitive and detect over a broad range of frequencies, for example, superconducting hot-electron bolometers, can be easily saturated by unwanted background signals which occur at frequencies different from the frequency of the desired signal.
(4) Cooling: All sensitive detectors of Terahertz-frequency radiation must be cooled to reduce the effects of thermal fluctuations. The amount of cooling required to reach the sensitivity required for a given application is a critical consideration for that application.
(5) Combining high speed and high sensitivity: There is usually a trade-off between high speed and high sensitivity. For example, the most sensitive bolometers have noise-equivalent powers (NEPs) of order 10-16 W/Hz.sup.1/2 and time constants of several milliseconds. See, D. C. Alsop, et.al. "Design and construction of high-sensitivity, infrared bolometers for operation at 300 mK," Applied Optics 31 (31), 6610-15 (1992). For many applications, it is essential to have a detector with both high speed and high sensitivity.
What is needed is a detector which overcomes each of the foregoing shortcomings.